Desalination is a separation process used to reduce the dissolved salt content of saline water to a usable level. The saline feedwater is drawn from the sea or from underground sources (brackish water). The desalination process separates it into two output streams: low-salinity product water and very saline concentrate stream (brine or reject water). The product water of the desalination process is generally water with less than 0.5 g/L or 500 ppm TDS (Total Dissolved Solids), which is suitable for most domestic, industrial, and agricultural uses. A by-product of desalination is brine, which is a concentrated salt solution and is usually disposed of.
A typical membrane process used for desalination is the reverse osmosis (RO). As shown schematically in FIG. 1, in the RO process, saline feedwater 10 is pressurized by a pump 12 and fed into a pressure vessel 14. Pretreatment agent 16 may be added to condition the feedwater. RO membranes 18 are disposed in the vessel 14 across the flow path. The membranes 18 inhibit the passage of dissolved salts while permitting the desalinated product water (also called permeate) 20 to pass through. The permeate 20 is forced to flow through the membrane by the pressure differential created between the pressurized feedwater and the product water, which is at near-atmospheric pressure. Because no membrane is perfect in its rejection of dissolved salts, a small percentage of salt passes through the membrane and remains in the product water. The remaining feedwater is discharged through the pressurized side of the pressure vessel as brine 22.
Boron is contained in seawater in a typical concentration of about 4–5 ppm. In one RO desalination stage, its concentration is lowered to 1÷1.5 ppm, which is tolerated by humans. Some agricultural crops, however, are troubled with boron toxicity at low levels. For example, California standard for irrigation water is a maximum of 0.75 mg/L (0.75 ppm). In Israel, where the desalinated water is used in a unified system both for potable water and for irrigation, requirements to boron content are even more rigorous —0.2 ppm (0.2 mg/L). Some industrial applications, such as the manufacture of electronic parts, speciality foods, and pharmaceuticals, also require very low concentration of boron in water.
At present, such standards can only be met by the use of costly ion exchange resins. Membrane systems have failed to perform effectively due to scaling problems or insufficiently high rejection rates for boron. The above described reverse osmosis method allows to remove only 60% to 80% of boron ions, while the other ions are removed by more than 99%, usually 99.6%.
The RO membranes have low effect in separating boron ions at pH<9. This fact is due to some peculiarities in the dissociation of various boric acid forms in seawater. It is known that boron ions rejection by reverse osmosis membranes increases with increasing feedwater pH. However, seawater desalination by the RO method is not practical at pH>9 due to the crystallization of CaCO3 and Mg(OH)2 salts on the RO membranes (fouling).
The improvement of the RO membrane method for removing the boron ion from desalinated seawater has been addressed by a number of inventors. Prior art JP 11138165 suggests treatment of the feed water with antiscaling agent before RO processing at pH=9.2. U.S. Pat. No. 5,250,185 suggests removal of all bivalent cations (such as Mg++) from feed water by treatment with water softener prior to RO process at pH>9.5. U.S. Pat. No. 5,925,255 suggests removal of hardness and non-hydroxide alcalinity from feed water in a weak acid cation ion exchange resin, then RO processing at pH>10.5.
Suggestions have been made to remove the boron ions before the RO treatment. JP 09220564 suggests adding a floculant to feed water, forming boron-containing insoluble precipitate, and microfiltering. Similarly, JP 10225682 suggests adding a coagulant for the same purpose. JP 10080684 suggests adding sodium fluoride to boron-containing feed water in order to form a complex, which is then rejected by a RO membrane.
According to other suggestions, boron ions are removed after the RO desalination. JP 11128922 suggests treating the boron-containing permeate in one or two positively charged RO membranes. JP 11128923 removes the boron ion in a series of RO membranes. JP 10015356, JP 10085743, and JP 11128924 use various types of ion exchange devices after the RO membrane. However, this group of inventions does not address the membrane scaling at high pH.
Still other suggestions deal with processes with two or more stages of RO for successive removal of scaling salts and boron ions. JP 59213489 suggests adding chlorine agent to boron-containing feedwater, and a first-stage RO treatment through chlorine-resistant membrane to remove inorganic and organic salts, Ca and Mg salt, and a part of boron. Then the permeate undergoes a second stage separation by a permeable membrane having an N-identical bond in the —CONH—molecule. JP 9290275 suggests removing polivalent cations in a first stage RO device, then raising the alcalinity of the permeate to pH≧9 by adding lime and removing boron in a second stage RO device. JP 8206460 combines a first stage RO unit separating polivalent ions at pH=6.5, a second stage RO unit for separation of the first-stage permeate at pH=9.5 to 11, and a third-stage RO unit for separation of the second-stage brine. JP 11010146 suggests a two-stage RO separation at neutral pH, where the first stage permeate is collected in two flows, a first one from the high-pressure feed side of the pressure vessel, and a second one from the brine discharge side. The second flow is passed through a second stage RO and then the second stage permeate is mixed with the first flow.
JP 11267645 discloses a two-stage RO desalination method. The boron containing feed water is acidified, deaerated, and passed through a first stage RO device. Then the water is conditioned to pH≧9.2 and treated in a second stage RO device.